The long term aim of this research project is the clarification of the mechanisms by which RNA tumor viruses cause neoplasia. The immediate goal is to verify a number of assumptions related to the origin of transforming genes in avian sarcoma viruses, and test the validity of the hypothesis that such genes are of cellular origin. The method suggested here to investigate these problems is a genetic analysis of Rous sarcoma virus (RSV) - chick embryo cell interaction. This research will involve: 1) the isolation of temperature sensitive (TS) mutants from different strains of RSV. 2) Physiological characterization of the mutants. 3) Complementation and recombination studies with these mutants. 4) Induction of tumors in birds by chemical carcinogens and non-transforming derivatives of RSV and attempt to isolate "transforming" viruses or rescue "transforming" genes. It is felt that the genetic analysis of the RSV - chick embryo fibroblast model system will help to clarify the mechanisms of tumor formation by RNA tumor viruses and to elucidate several aspects of the more general problem of viral carcinogenesis.